The object of the investigation, of which the present paper forms a part, is 1. To determine the relative composition of the various resins which occur in nature. Possessing so many properties in common, this large family of natural productions ought also to present many analogies in constitution. 2. To ascertain how far they may be considered as derivatives from one common radical; and 3. Whether it is possible to represent them all by one or more general formulæ. I. Resin of Mastic. Mastic resin is said to be obtained from the Pistacea lentiscus , and to be produced chiefly in the island of Chios. It occurs in drops or tears, which are transparent, and of a pale yellow colour. It melts at 212°, and emits a peculiar and not unpleasant odour. Fused in a retort it gives off an acid liquid in small quantity. If the heat be raised to 300° Fahr. and upwards, the melted mass froths up, and water and acid vapours are evolved. At a higher temperature a pale yellow liquid distils over very slowly, at first of the consistence of oil, but increasing in thickness as the process proceeds, water and acid being also given off during the whole process. What remains in the retort is of a black colour, and nearly insoluble in alcohol.